Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-06-26
2004-10-26
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S458000, C568S492000, C568S853000, C568S862000, C568S863000
Reexamination Certificate
active
06809224
ABSTRACT:
The present invention relates to the area of industrial organic chemistry. More precisely, the present invention relates to a process for effectively removing formaldehyde from methylolalkanal solutions. The present invention furthermore relates to a process for hydrogenating methylolalkanals thus obtained to give polyols.
The condensation of formaldehyde with CH-acidic higher alkanals to give methylolalkanals, in general dimethylol- and trimethylolalkanals, and conversion of the compounds obtained to polyols constitute a widely used process in chemistry. Examples of important triols obtained in this manner are trimethylolpropane, trimethylolethane and trimethylolbutane, which are widely used for the production of finishes, urethanes and polyesters. Other important compounds are pentaerythritol, obtainable by condensation of formaldehyde and acetaldehyde, and neopentylglycol from isobutyraldehyde and formaldehyde. The tetrahydric alcohol pentaerythritol is also frequently used in the coatings industry but is also very important in the production of explosives.
The polyols can be prepared by various processes. One method is the Cannizzaro process, which is further subdivided into the inorganic and the organic Cannizzaro process. In the inorganic variant, an excess of formaldehyde is reacted with the corresponding alkanal in the presence of stoichiometric amounts of an inorganic base, such as NaOH or Ca(OH)
2
. The methylolalkanal formed in the first stage reacts in the second stage with the excess formaldehyde in a disproportionation reaction to give the corresponding polyol and the formate of the corresponding base, i.e. for example sodium formate or calcium formate. The production of these salts is a disadvantage since they are difficult to separate from the reaction product and moreover one equivalent of formaldehyde is lost.
In the organic Cannizzaro process, a tertiary alkylarnine is used instead of an inorganic base. Consequently, higher yields can be achieved than with an inorganic base. Trialkylanmonium formate is obtained as an undesirable byproduct. Thus, here too, one equivalent of formaldehyde is lost.
The disadvantages of the Cannizzaro process are avoided in the hydrogenation process. Here, formaldehyde is reacted with the corresponding alkanal in the presence of catalytic amounts of an amine. Consequently, the reaction stops at the methylolalkanal stage. After removal of the formaldehyde, the reaction mixture, which still contains small amounts of the corresponding triol in addition to the corresponding alkanal, is subjected to a hydrogenation in which the desired polyol is obtained.
Different variants of these hydrogenation processes are described, inter alia, in the applications DE-A-25 07 461, DE-A-27 02 582, DE-A-27 14 516, DE-A-28 13 201, DE-A-33 40 791 and WO 98/28253.
A major problem of the hydrogenation process is effective removal of the formaldehyde used in excess, after the condensation reaction, before the use of the reaction mixture in the hydrogenation. Effective removal of the formaldehyde is desirable simply because it can be recycled to the reaction and is thus not lost. In principle, it would be possible to hydrogenate the reaction mixture directly after the condensation without prior removal of the formaldehyde. However, this would be converted into the relatively useless methanol; furthermore, an increased consumption of hydrogen and a higher catalyst loading would also be the result, so that an unfavorable balance would be obtained in the end.
The literature describes various methods for effectively separating formaldehyde from the reaction mixtures of the condensation. In connection with the present invention, the references cited below are of interest.
DD-A-273 434 describes a process for separating excess formaldehyde from reaction solutions, in which formaldehyde is reacted with its higher alkanals by the classical Cannizzaro process. A crude mixture containing the polyhydric alcohol and the formaldehyde used in excess is obtained. Formaldehyde is separated by addition of methanol, water and/or acetonitrile and subsequent azeotropic distillation of the formaldehyde as a complex mixture with the added solvents and further components. The polyhydric alcohol remains in the bottom.
Japanese Patent JP 10287606 (cited according to CA 1998/129: 275637) describes a process for removing formaldehyde from reaction mixtures which originate from the reaction of this aldehyde with higher aldehydes and contain the corresponding dimethylolalkanal as a product. The removal is effected by distillation in a thin-film evaporator. Before the distillation, water is added to the mixture, in amounts of >4 parts by weight. As a result of the distillation, a residual formaldehyde content of 5% is reached, at pressures of from 0.5 to 1 bar.
U.S. Pat. No. 4,036,888 also describes the addition of water to reaction mixtures of the reaction of formaldehyde with higher aldehydes and subsequent distillation. In this case, isobutyraldehyde is used and the product is hydroxypivalaldehyde. By adding water, the isobutyraldehyde used in excess is effectively removed here. For this reason, the problem of removing formaldehyde does not exist here since it is present after the reaction only in very small amounts.
The same also applies to U.S. Pat. No. 5,235,118, which relates to the reaction of 2-ethylhexanal with formaldehyde to give 2-ethyl-2-(hydroxymethyl)hexanal. Here too, formaldehyde is used in substoichiometric amounts relative to the 2-ethylhexanal, and only a small residue of formaldehyde is present in the product solution. The addition of water with subsequent distillation serves for effective removal of unreacted 2-ethylhexanal.
It is an object of the present invention to provide a process which permits effective removal of formaldehyde from the methylolalkanals obtained after reaction of formaldehyde with higher aldehydes. The separation should be so effective that the subsequent hydrogenation of the methylolalkanal is economical.
We have found that this object is achieved by a process for the removal, by distillation, of formaldehyde from reaction solutions containing a methylolated alkanal which was obtained from the reaction of formaldehyde with an alkanal which has at least one acidic hydrogen atom ax to the carbonyl function or from the reaction of a 2-alkylacrolein or acrolein with water and formaldehyde, this reaction having been carried out in the presence of catalytic amounts of an organic amine, wherein a suitable amount of water is added before and/or during distillation.
We have found that this object is furthermore achieved by a process for the preparation of polyols, wherein a methylolated alkanal which was freed from formaldehyde according to the process described above is subjected to a hydrogenation known per se.
It was found that effective separation can be achieved by adding water before and/or during the removal of formaldehyde by distillation from the methylolalkanal obtained after the condensation of formaldehyde with a higher aldehyde. Furthermore, the yield in a subsequent hydrogenation can be increased by adding water.
In general, the methylolalkanals will be di- or trimethylolalkanals. The methylolalkanals are generally prepared by condensation of the abovementioned starting compounds in the presence of catalytic amounts of an organic amine. In the present process, it is possible in general to use virtually all formaldehyde-containing discharges from an aldol condensation which are prepared by said condensation reaction. The process can be particularly advantageously used in the case of discharges from an aldol condensation which were obtained according to WO 98/28253. The content of this application is hereby incorporated by reference in the present application.
The removal of the formaldehyde from the aldol condensation mixtures is effected in a suitable distillation apparatus. Such apparatuses are known to those skilled in the art. It is preferable to use a distillation column.
Suitable distillation columns have from 3 to 100 theore
Dernbach Matthias
Kratz Detlef
Schulz Gerhard
Stammer Achim
BASF - Aktiengesellschaft
Keil & Weinkauf
Keys Rosalynd
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